Focusing an image capture instrument consists in bringing about the coincidence of a plane in which an image is formed by an optical system, with an image detection plane in which a photosensitive surface of at least one image sensor is situated.
In the cameras intended for the consumer market, the image formation optical system is a lens in which the focal length can be adjusted by moving at least one of the lenses. The image sensor is fixed and optical focusing consists in moving the mobile lens until the image formation plane is superimposed on the photosensitive surface of the image sensor. Optical focusing can then be used to acquire sharp images of scenes captured with variable distances. Several automatic focusing, or “autofocus,” methods are known. In these methods, several test images are acquired successively before the final image by intentionally modifying the lens adjustment. All the test images contain a shared portion of the optical field of the final image, which can be all or part of this optical field. An analysis of the test images is used to obtain a relative estimate of the defect in the optical focus that exists for each of them. An estimate of the adjustment that will produce the focus is then extrapolated, or a progressive approach to this adjustment is used, or the two methods are combined to more rapidly obtain the final optical focus adjustment. The final image is then acquired by using the focus adjustment that has thus been determined.
Another field of imaging is that whereby an image is captured by satellites or spacecrafts. In this case, the image formation optics may be a telescope and the photographed scene is always very distant, so that the imaging plane is the focal plane of the telescope. But the position of the focal plane can vary, especially because of thermo-elastic deformations that affect the optics, even more so when the dimensions of the optics are large. Various optical focusing systems that have been adapted to spatial conditions and to the large dimensions of the optical components used are, therefore, implemented. However, the methods used for mass-market photography are not well suited to the requirements of spatial imaging, when characterizing a focusing defect that exists for a given adjustment of the image capture instrument. Mainly, the satellite or spacecraft onboards which the instrument is loaded can move rapidly with respect to the scene photographed, so that it is difficult to successively capture several test images of the same scene, and very disadvantageous in terms of productivity.
The article entitled “Realization of Imaging-Auto-Focus on the APRC Using Splicing-CCD” by Zhenhua Lu et al., Proc. of SPIE, vol. 8285, 82854L-1 (International Conference on Graphic and Image Processing, 2011), describes an optical focusing method that is specifically adapted to the known method of image capture known as “push-broom.” In this method, a series of images are successively captured as a band of the optical field, known as the scan swath, is swept over a longitudinal direction. The scanning of the scan swath occurs as the satellite travels above the scene. The method described in this article uses several image sensors arranged in two rows within a shared image capture plane, the two rows being perpendicular to the apparent motion of the scene in this plane. The method is based on the following conditions:                detectors each in a different row have overlapping portions of their respective photosensitive surfaces, in projection on the direction of the rows;        two images are successively captured in synchronized manner using across-track scanning, so that a same portion of the scene is captured in the image, first by a part of a detector of the first row, then by a part of detector of the second row; and        adjustment of optical focus is varied between the two images, to then compare the different focusing defects that respectively affect the two images of the same portion of a scene. A modification of the focus can then be determined, at least a direction of such modification, to reduce the focusing defect.        
Such method therefore requires the activation of an optical focusing system between two images successively captured during the scan. However, optical focusing systems that are compatible with the conditions of use on-board a satellite or spacecraft are often slow, so that the method is not well suited for rapid scans.
For the same reason, whenever scanning is too rapid for a given method to be implemented, the two images of the same scene are limited to portions of the photosensitive surface of the image sensors that correspond to approximately 1,000 lines of picture points, or pixels, by counting the lines perpendicularly to the direction of the rows of sensors. Yet, many methods for characterizing optical focusing defects are based on a statistical evaluation of the differences between intensities that are individually captured by pixels. The optical focusing defect can, therefore, be determined with greater reliability when each of the two images of the same scene has a large number of pixel rows.